Diffusion transfer photography is well known and has been commercially utilized to provide "instant" images in both color and black and white. Color films are available which provide reflection prints in a peel-apart format, e.g., Type 108 Polaroid Polacolor 2 color film, or in an integral format in which the image component remains bonded to the photosensitive component, e.g., Polaroid SX-70 Land film or Kodak PR-10 film.
Black and white transfer images are formed by the imagewise transfer of complexed silver from the undeveloped areas of the exposed silver halide emulsion to an image-receiving layer containing silver precipitating agents or nuclei. Films are commercially available which provide a silver transfer image in a peel-apart format, e.g., Type 107 Polaroid Land film, but as yet no integral black and white silver transfer film has been commercialized, although references to such a film may be found in the patent literature.
One of the problems encountered in formulating an integral silver transfer reflection print has been the need to provide an opaque layer, i.e., a layer of carbon black, between the silver halide emulsion layer and the image-receiving layer so that the film unit may be developed outside the camera. In addition, a light-reflecting layer, e.g., a white layer of titanium dioxide, is positioned between the carbon black layer and the image-receiving layer to provide a white background against which the silver transfer image may be viewed. The complexed silver has to diffuse from the silver halide emulsion layer through the layer of carbon black and the titanium dioxide layer to reach the image-receiving layer where the silver is precipitated to provide the desired image. For reasons not completely understood, the carbon black appears to prevent a significant portion of the diffusing silver complex from reaching the image-receiving layer. The resulting silver transfer image thus may exhibit an undesirably low maximum density. While the silver transfer density may be increased, for example, by coating more silver halide, this is an undesirable alternative because of the significant added cost. In addition, the additional silver halide may give rise to other sensitometric problems.